Abstract:

Surface texturing is employed to modify the topography of one or more
surfaces (radial or cylindrical) of the sealing system for a roller cone
rock bit. The surface texturing results in a dimpled surface which
retains additional lubricant helpful in reducing friction in the boundary
and mixed lubrication regimes. Shot peening is disclosed as one method
for texturing the desired surface.

Claims:

1. A method for surface texturing surfaces of a sealing system,
comprising:exposing a sealing surface of interest to a first shot peening
action wherein the sealing surface is bombarded at a first intensity
level by first small spherical media of a first average size; andexposing
the sealing surface of interest to a subsequent second shot peening
action wherein the surface is bombarded at a second intensity level by
second small spherical media of a second average size.

2. The method of claim 1 wherein the second intensity level is reduced
from the first intensity level.

3. The method of claim 1 wherein the second average size is smaller than
the first average size.

4. The method of claim 1 wherein the second intensity level is reduced
from the first intensity level and wherein the second average size is
smaller than the first average size.

5. The method of claim 1 further comprising exposing the sealing surface
of interest to one or more additional shot peening actions.

6. The method of claim 1 wherein the first shot peening action surface
textures between 60-100% of the surface of interest.

7. The method of claim 6 wherein the second shot peening action surface
textures between 60-100% of the surface of interest.

8. The method of claim 1 wherein each of the first and second shot peening
actions surface textures 100% of the surface of interest.

9. The method of claim 1 wherein, following completion of the second shot
peening action, 60-100% of the sealing surface of interest has been
textured with randomly sized and distributed dimples.

10. A method for building a drill bit, comprising:forming a bearing
structure having a sealing system including at least one sealing surface
adjacent to a seal; andsurface texturing the sealing surface of the
sealing system, wherein surface texturing comprises:exposing the sealing
surface to a first shot peening action wherein the surface is bombarded
at a first intensity level by first small spherical media of a first
average size; andexposing the sealing surface a subsequent second shot
peening action wherein the surface is bombarded at a second intensity
level by second small spherical media of a second average size.

11. The method of claim 10 wherein the sealing surface is one of either a
cylindrical or conical sealing surface on a shaft of the journal bearing
system.

12. The method of claim 10 further including surface texturing a bearing
surface of the bearing structure according to the two exposing steps.

13. The method of claim 12 wherein the bearing structure includes a
bearing bushing and the bearing surface that is surface textured is one
of an inside or outside cylindrical surface of the bushing.

14. The method of claim 12 wherein the bearing structure includes a
bearing shaft and the bearing surface that is surface textured is an
outer cylindrical surface of the shaft.

15. A sealing system for a bearing of a rock bit having a sealing surface
adjacent a sliding seal, wherein the sealing surface has a surface
texture comprised of a plurality of dimples.

16. The sealing system of claim 15 wherein the dimples cover between 5-60%
of the sealing surface.

17. The sealing system of claim 15 wherein the dimples cover between
60-100% of the sealing surface.

18. The sealing system of claim 15 wherein the plurality of dimples are
formed by shot peening in a two step process including a first shot
peening action wherein the sealing surface is bombarded at a first
intensity level by first small spherical media of a first average size
and a subsequent second shot peening action wherein the sealing surface
is bombarded at a second intensity level by second small spherical media
of a second average size.

19. The system of claim 18 wherein the first shot peening action surface
textures between 5-100% of the surface of interest.

20. The system of claim 19 wherein the second shot peening action surface
textures between 5-100% of the surface of interest.

21. The system of claim 18 wherein each of the first and second shot
peening actions surface textures 100% of the surface of interest.

22. The system of claim 15 wherein the sealing surface is a surface
associated with a seal gland.

23. The system of claim 15 wherein the sealing surface is one of either a
cylindrical or conical sealing surface.

24. The system of claim 15 further wherein the bearing includes a bearing
bushing, and wherein one of an inside or outside cylindrical surface of
the bushing has a surface texture comprised of a plurality of dimples
which cover at least 60% of the surface of interest.

25. The system of claim 15 wherein the bearing includes a bearing shaft
and wherein an outer cylindrical surface of the shaft has a surface
texture comprised of a plurality of dimples which cover at least 60% of
the surface of interest.

26. A bearing system for a bearing of a rock bit, comprising:a shaft
including a seal surface and a cylindrical bearing surface;a bushing;a
roller cone having a annular gland and which receives the bushing;wherein
the roller cone and bushing are rotatably mounted to the shaft such that
the bushing aligns with the cylindrical bearing surface and the gland
aligns with the seal surface; andwherein at least both the seal surface
and at least one of a) the cylindrical bearing surface on the shaft or b)
an inner cylindrical surface of the bushing have a surface texture, that
surface texture including a plurality of dimples.

27. The bearing system of claim 26 wherein the dimples cover, with respect
to both the seal surface and the bearing surface, between 60-90% of the
surface.

28. The bearing system of claim 26 wherein the dimples cover, with respect
to both the seal surface and the bearing surface, substantially 100% of
the surface.

29. The bearing system of claim 26 wherein the plurality of dimples are
formed by shot peening in a two step process including a first shot
peening action wherein the textured surface is bombarded at a first
intensity level by first small spherical media of a first average size
and a subsequent second shot peening action wherein the textured surface
is bombarded at a second intensity level by second small spherical media
of a second average size.

30. The bearing system of claim 26 wherein the cylindrical bearing surface
is radially offset from the seal surface.

31. The bearing system of claim 26 wherein the sealing surface is one of
either a cylindrical or conical sealing surface on the shaft.

32. The bearing system of claim 26 wherein the dimples cover, with respect
to the seal surface, less than 60% of the surface, and cover, with
respect to the bearing surface, between 60-100% of the surface.

33. A method for building a drill bit, comprising:forming a bearing
structure having a sealing system including at least one sealing surface
adjacent to a seal; andsurface texturing the sealing surface of the
sealing system, wherein surface texturing comprises exposing the sealing
surface to a shot peening treatment wherein the surface is bombarded at a
first intensity level by first small spherical media of a first average
size.

34. The method of claim 33 wherein surface texturing further comprises
exposing the sealing surface to a subsequent second shot peening action
wherein the surface is bombarded at a second intensity level by second
small spherical media of a second average size.

35. The method of claim 33 wherein the sealing surface is one of either a
cylindrical or conical sealing surface on a shaft of the journal bearing
system.

36. The method of claim 33 further including surface texturing a bearing
surface of the bearing structure, the texturing of the sealing surface
being lighter than the surface texturing of the bearing surface.

Description:

PRIORITY CLAIM

[0001]This application claims the benefit of U.S. Provisional Application
for Patent Ser. No. 61/036,762 filed Mar. 14, 2008, the disclosure of
which is hereby incorporated by reference.

[0003]A roller cone rock bit is the cutting tool used in oil, gas, and
mining fields to break through the earth formation to shape a well bore.
Load and motion of the bit are transferred to the bearings inside three
head and cone assemblies. For the bit where a journal bearing is
employed, the main journal bearing is charged with as much as 80 percent
of the total radial load. The main journal bearing is composed of the
head (as the shaft), the bushing, and the cone (as the housing). This
bearing is lubricated and sealed. An outer circumference of the seal is
compressed by a gland of the cone so that the seal moves together with
the cone and slides against the head (at a sealing surface or seal boss)
on the inner circumference of the seal. The seal is thus confined in the
seal gland to secure the lubricant within the bearing and prevent debris
from invading into the bearing. The longer the seal excludes
contamination from the bearing, the longer the bearing life. Therefore,
the seal can become the limiter of the rock bit life.

[0004]An elastomer seal is known in the prior art as the dominant sealing
element in rock bits. Various types of elastomer seals have been
developed. The seal is very flexible, and is compatible with the drilling
mud. The seal has excellent resilience at relatively high temperatures.
Thus, the seal has proved to be sufficient to provide enough sealing
force to separate the mud and debris environment from the lubricant over
an acceptable period of time.

[0005]However, friction between the seal and surfaces of the seal gland as
the cone rotates can cause damage to the seal itself. Over time, this
damage accumulates to the point where the seal itself fails. Following
seal failure, the bearing experiences grease starvation in the contact
zone due to loss of lubricant in the bearing system. Thereafter,
excessive wear appears on the bearing system surfaces due to shearing and
heating caused by sliding friction. The end result is typically scoring,
scuffing, and even catastrophic failure like galling or seizure. It is
thus imperative that lubrication be retained between contact interface
surfaces of the journal bearing. Maintaining seal life is thus critical
to maintaining bit life.

[0006]One way to extend seal life is to reduce the friction between seal
and head. Under typical running conditions, the seal experiences mixed
lubrication. In this lubrication regime, more lubricant is necessary at
the contact point between the seal and one or more of the gland surfaces
in order to reduce the friction. Thus, there is a need in the art to
introduce more lubricant in the sealing zone.

[0007]Reference is made to FIG. 1 which illustrates a partially broken
away view of a typical roller cone rock bit. FIG. 1 more specifically
illustrates one head and cone assembly. The general configuration and
operation of such a bit is well known to those skilled in the art.

[0008]The head 1 of the bit includes the bearing shaft 2. A cutting cone 3
is rotatably positioned on the bearing shaft 2 which functions as a
journal. A body portion of the bit includes an upper threaded portion
forming a tool joint connection 4 which facilitates connection of the bit
to a drill string (not shown). A lubrication system 6 is included to
provide lubrication to, and retain lubricant in, the journal bearing
between the cone 3 and the bearing shaft 2. This system 6 has a
configuration and operation well known to those skilled in the art.

[0009]A number of bearing systems are provided in connection with the
journal bearing supporting rotation of the cone 3 about the bearing shaft
2. These bearing systems include a first cylindrical friction bearing 10
(also referred to as the main journal bearing herein), ball bearings 12,
second cylindrical friction bearing 14, first radial friction (thrust)
bearing 16 and second radial friction (thrust) bearing 18.

[0010]FIG. 2 illustrates a partially broken away view of FIG. 1 showing
the bearing system and sealing system in greater detail. The first
cylindrical friction bearing 10 is defined by an outer cylindrical
surface 20 on the bearing shaft 2 and an inner cylindrical surface 22 of
a bushing 24 which has been press fit into the cone 3. This bushing 24 is
a ring-shaped structure typically made of beryllium copper, although the
use of other materials is known in the art. The ball bearings 12 ride in
an annular raceway 26 defined at the interface between the bearing shaft
2 and cone 3. The second cylindrical friction bearing 14 is defined by an
outer cylindrical surface 30 on the bearing shaft 2 and an inner
cylindrical surface 32 on the cone 3. The outer cylindrical surface 30 is
inwardly radially offset from the outer cylindrical surface 20. The first
radial friction bearing 16 is defined between the first and second
cylindrical friction bearings 10 and 12 by a first radial surface 40 on
the bearing shaft 2 and a second radial surface 42 on the cone 3. The
second radial friction bearing 18 is adjacent the second cylindrical
friction bearing 12 at the axis of rotation for the cone and is defined
by a third radial surface 50 on the bearing shaft 2 and a fourth radial
surface 52 on the cone 3.

[0011]With respect to the sealing system, an o-ring seal 60 is positioned
between cutter cone 3 and the bearing shaft 2. A sealing surface, for
example, a cylindrical surface seal boss 62, is provided on the bearing
shaft. In the illustrated configuration, this sealing surface provided by
the seal boss 62 is cylindrical and outwardly radially offset (by the
thickness of the bushing 24) from the outer cylindrical surface 20 of the
first friction bearing 10. It will be understood that the sealing surface
(of the seal boss 62 for example) could exhibit no offset with respect to
the main journal bearing surface, or be inwardly radially offset, if
desired. Additionally, it will be understood that the sealing surface
(62) need not be cylindrical but rather may be conical if desired. An
annular groove is formed in the cone 3 to define the seal gland 64. The
groove and sealing surface (seal boss 62) align with each other when the
cutting cone 3 is rotatably positioned on the bearing shaft to define the
gland 64 region. The o-ring seal 60 is compressed between the surface(s)
of the gland 64 and the sealing surface (seal boss 62), and functions to
retain lubricant in the bearing area around the bearing systems and
prevents any materials (drilling mud and debris) in the well bore from
entering into the bearing area.

[0012]Load in the bearing system is supported by both asperity contact and
hydrodynamic pressure. Lubricant is provided in the first cylindrical
friction bearing 10, second cylindrical friction bearing 14, first radial
friction bearing 16 and second radial friction bearing 18 between the
implicated cylindrical and radial surfaces using the system 6. Lubricant
is retained in the bearing system by the compressed seal 60 in the gland
64. That lubricant not only lubricates the bearing system, but also
provides a measure of lubricant on the surfaces of the seal gland 64, and
especially the sealing surface such as the seal boss 62 surface, that
assists in allowing the compressed seal 60 to slide along the sealing
surface (for example, seal boss 62 outer cylindrical surface) as the cone
rotates.

[0013]The seal is designed to withstand a high pressure in downhole
drilling applications. That high pressure, together with a designed high
compression rate of the seal in gland, compresses the seal tightly
against the seal boss 62. The lubricant which is present in the sealing
zone at the seal boss surface provides lubrication to the seal and takes
away friction heat. In the case where the seal is not well lubricated, it
slides dry against the seal boss and a large amount of friction heat is
generated. This friction heat is known to be the root cause of seal
failure. It is accordingly desirable to introduce more lubricant
underneath the seal, such as on the seal boss 62 surface (or other
sliding gland surface), in order to reduce friction and carry away heat.

[0014]It is not unusual for the bearing to experience grease starvation in
these surface contact zones of the bearing system. This can result in
scoring, scuffing, and even catastrophic failure like galling or seizure
of the journal bearing. There is accordingly also a need to retain
lubricant in position trapped between the implicated and opposed
cylindrical and radial surfaces of the bearing system.

[0016]To address issues of grease starvation and possible seal failure, it
is desired to increase the amount of lubricant that can be maintained in
the surface contact zones of the sealing system. In an effort to
introduce more lubricant into these surface contact zones, the surface
topography of the sealing system (for example, seal gland surfaces) is
modified in the manner described below.

[0017]Surface texturing is employed to modify the topography of one or
more surfaces (radial, conical or cylindrical) of the sealing system for
a rock bit. Innovative methods and apparatus are described with respect
to certain features of surface texturing and its beneficial effect on
reducing friction and prolonging bit life. These features address
deficiencies of the prior art with respect to the configuration and
operation of the sealing surfaces.

[0018]In accordance with an embodiment, the surface topography of the
sealing system is modified by surface texturing technology. A surface
texture is introduced, preferably on the seal boss cylindrical surface at
the seal location, to the sealing system for the roller cone rock bit.
The surface texturing disclosed herein includes dimples which retain
additional lubricant and are thus helpful to reduce friction at the seal.

[0019]Surface texturing as described herein creates specially patterned
dimples on one or more surfaces of the sealing system. With reference
once again made to FIGS. 1 and 2, the textured surface in the sealing
system in accordance with embodiments described herein is preferably the
seal boss 62 surface. It will be understood, however, that depending on
the configuration of the gland 64, one or more other surfaces associated
with defining the gland and compressing the seal 60 could have a surface
texturing as well. Thus, any desired surface, including cylindrical,
conical and radial surfaces, of the gland 64 area for the sealing system
could possess a surface texturing Further, any combination of textured
surfaces, with untextured surfaces, may be used in the sealing system.

[0020]To address issues of grease starvation and possible bearing failure,
it is also desired to increase the amount of lubricant that can be
maintained in the surface contact zones of the bearing system. In an
effort to introduce more lubricant into these surface contact zones, the
surface topography of the bearing system is modified in the manner
described below. Surface texturing is employed to modify the topography
of one or more surfaces (radial or cylindrical) of the bearing system for
a rock bit.

[0021]Due to heavy load and low velocity, the head shaft and the bushing
of the main journal bearing are in contact at the loading side of the
bearing system. This metal-to-metal contact dominates the frictional
behavior of the bearing system. The friction coefficient can normally
reach over 0.1, which generates enormous heat and can lead to bearing and
seal failure. To improve the bearing life, the friction has to be
reduced. In a mixed lubrication regime, there are two means to create
better lubrication in these surface contact areas: supply more grease or
generate greater hydrodynamic pressure.

[0022]In accordance with an embodiment, the topography of the head bearing
system is modified by surface texturing technology. A surface texture is
introduced, either on the head side or on the bushing (or cone) side (or
both), of the bearing system for the roller cone rock bit. The surface
texturing disclosed herein includes dimples which retain additional
lubricant and are thus helpful to reduce the friction in the boundary and
mixed lubrication regimes.

[0023]Surface texturing as described herein creates specially patterned
dimples on one or more surfaces of the bearing system. Reference is once
again made to FIGS. 1 and 2 for an identification of possible textured
surfaces in the bearing system in accordance with embodiments described
herein. In one implementation, the surface texturing is provided on an
outer cylindrical surface 20 of the bearing shaft 2 which forms part of
the first cylindrical friction bearing 10. In another implementation, the
surface texturing is provided on an inner cylindrical surface 22 of the
bushing 24 which has been press fit into the cone 3 and which forms part
of the first cylindrical friction bearing 10. In yet another
implementation, the surface texturing is provided on an outer cylindrical
surface 30 of the bearing shaft 2 which forms part of the second
cylindrical friction bearing 14. In still another implementation, the
surface texturing is provided on an inner cylindrical surface 32 of the
cone 3 which forms part of the second cylindrical friction bearing 14. In
another implementation, the surface texturing is provided on a first
radial surface 40 of on the bearing shaft 2 which forms part of the first
radial friction bearing 16. In yet another implementation, the surface
texturing is provided on a second radial surface 42 of the cone 3 which
forms part of the first radial friction bearing 16. In still another
implementation, the surface texturing is provided on a third radial
surface 50 of the bearing shaft 2 which forms part of the second radial
friction bearing 18. In yet another implementation, the surface texturing
is provided on a fourth radial surface 52 of the cone 3 which forms part
of the second radial friction bearing 18. Any combination of the
foregoing textured surfaces, with untextured surfaces, may also be used.

[0024]The dimples of the surface texturing behave as lubricant reservoirs
which permeate the lubrication into the inter-space of metal asperities.
Meanwhile, higher hydrodynamic pressure is generated on the dimple area.
Both functions will facilitate an improvement in sealing or bearing
system lubrication with a reduction in friction.

[0025]The percentage coverage area with respect to the dimples may be the
same or different for the sealing surface and bearing surface. In one
implementation, the coverage is the same, and it is preferred that the
dimples of surface texture cover between 60-100% of the surface of
interest. Even more preferably, the dimples should cover between 70-90%
of the surface of interest. In an implementation, the dimples cover
substantially 100% of the surface of interest. In another implementation,
the sealing surface may have a lighter shot peen than the bearing
surface. So, the bearing surface texture coverage may have the
percentages as described previously, while the sealing surface has a
lower texture coverage area. Examples of the lower surface texture
coverage include 5-60%.

[0026]Embodiments herein utilize any one or more of a variety of methods
to create surface texturing including: machining, chemical etching, laser
texturing, deep rolling, vibratory finishing, etc. Controllability,
uniformity, cost, coverage area, dimple size, dimple depth, and dimple
shape are the factors which determine which method is selected to form
the texturing.

[0027]In a preferred implementation, shot peening is used to create the
dimples of the surface texturing. More specifically, a two-step shot
peening process is used. In accordance with this two-step process, in a
first step the sealing or bearing system surface to be treated is exposed
to a first shot peening action wherein the surface is bombarded at a
first intensity level by small spherical media (the "shot") of a first
average size. In a second step the same sealing or bearing system surface
being treated is exposed to a second shot peening action at a second
intensity level wherein the surface is bombarded by small spherical media
(the "shot") of a second average size. Preferably, the second intensity
level is reduced from the first intensity level. Preferably, the second
average size is smaller than the first average size.

[0028]In a preferred implementation, each step of the two-step shot
peening process is effectuated to achieve a peened coverage area of
between 60-100%. When peened coverage areas of less than 100% are used in
each step, the goal is to achieve a final peened coverage area with
respect to the treated surface of at least 60%, and more specifically
70-90% and even more preferably which approaches or reaches substantially
100%.

[0029]It will further be understood that the shot peening process could
utilize more than two separate peening actions. For example, a
three-step, four-step, or more-step process could be used. Each step
would preferably utilize different average sized media and different
intensity levels.

[0030]With respect to the lighter shot peen which may be used for the
sealing surface, it may only be necessary to utilize a single shot
peening action to achieve the desired surface texturing.

[0033]FIGS. 2A-2C illustrate other geometries for a sealing system used in
FIG. 2;

[0034]FIG. 3 illustrates an exemplary shot peening impact pattern;

[0035]FIG. 4 illustrates, in cross-section, a surface of interest which
has been treated by the shot peening of FIG. 3 at or about 100% coverage
area;

[0036]FIG. 5 illustrates, in cross-section, a surface of interest which
has been treated by additional shot peening with a coverage area of less
than 100%;

[0037]FIG. 6 illustrates an exemplary impact pattern with respect to
execution of the additional shot peening;

[0038]FIGS. 7, 8 and 9 illustrate, in cross-section, a surface of interest
which has been treated by an additional shot peening process;

[0039]FIGS. 10-12 are images illustrating topography comparisons between
surfaces that have subjected to a two-step shot peening process;

[0040]FIG. 13 is an image illustrating the topography of a regular
machined surface;

[0041]FIG. 14 is an image illustrating the topography of a two-step shot
peened surface; and

[0042]FIGS. 15-19 illustrate the beneficial effect on the friction
coefficient which accrues from surface texting.

DETAILED DESCRIPTION OF THE DRAWINGS

[0043]Surface texturing is employed to modify the topography of one or
more surfaces (radial, conical or cylindrical or other) of the sealing
and/or bearing system for a roller cone rock bit. The surface texturing
results in a dimpled surface which retains additional lubricant helpful
in reducing friction in the boundary and mixed lubrication regimes.
Surface coverage area for the dimpled texture, at least with respect to a
bearing surface, should exceed at a minimum 60%, more preferably be
between 70-90%, and even more preferably approach or reach approximately
100%. A lighter shot peen (5-60%) may be used in connection with a
sealing surface.

[0044]With reference to FIGS. 1 and 2, the textured surfaces in the
sealing system to which this surface texturing is applied preferably
comprise any surface having sliding contact with the seal 60 as the cone
rotates. This would include one or more surfaces of the gland 64. More
specifically, it would at least include the seal boss 62 surface. Any
combination of the foregoing textured surfaces, with desired untextured
surfaces, may also be used.

[0045]While FIGS. 1 and 2 illustrate the use of cylindrical sealing
surfaces associated with the boss 62 and gland 64, the surface texturing
may be applied to other geometries for the sealing system such as those
illustrated in FIGS. 2a, 2b and 2c. It will be noted that these
alternative geometries exploit conical surfaces in connection with the
sealing system (on either one of or both of the shaft and cone side of
the seal). Thus, any conical or cylindrical surface associated with the
seal and functioning as a sealing surface against which the seal 60 rides
as the cone rotates is a suitable candidate for texturing.

[0046]Reference is once again made to FIGS. 1 and 2 for an identification
of textured surfaces in the bearing system to which this surface
texturing is applied.

[0047]Turning first to the first cylindrical friction bearing 10, surface
texturing is provided on one or the other or both of the outer
cylindrical surface 20 on the bearing shaft 2 and the inner cylindrical
surface 22 of the bushing 24 which has been press fit into the cone 3,
these surfaces forming the first cylindrical friction bearing 10 (or main
journal bearing).

[0048]With respect to the second cylindrical friction bearing 14, surface
texturing is provided on one or the other or both of the outer
cylindrical surface 30 of the bearing shaft 2 and the inner cylindrical
surface 32 of the cone 3.

[0049]For the first radial friction bearing 16, surface texturing is
provided on one or the other or both of the first radial surface 40 of
the bearing shaft 2 and the second radial surface 42 of the cone 3.

[0050]Lastly, for the second radial friction bearing 18, surface texturing
is provided on one or the other or both of the third radial surface 50 of
the bearing shaft 2 and the fourth radial surface 52 of the cone 3.

[0051]Any combination of the foregoing textured surfaces, with desired
untextured surfaces, may also be used.

[0052]The dimples of the surface texturing behave as lubricant reservoirs
which permeate the lubrication into inter-space of metal asperities.
Meanwhile, higher hydrodynamic pressure is generated on the dimple area.
Both functions will facilitate an improvement in sealing and/or bearing
system lubrication.

[0053]Any one or more of a variety of methods can be used to create the
dimpled surface texturing including: machining, chemical etching, laser
texturing, deep rolling, vibratory finishing, shot peening, etc.
Controllability, uniformity, cost, coverage area, dimple size, dimple
depth, and dimple shape factors which influence which method is selected
for the surface texturing process.

[0054]The dimpled surface texture should be random and with uniform
coverage. Preferably, different sized dimples should be present and
should be randomly distributed across the surface. A finished coverage
area of substantially 100% on the surface of interest is preferred at
least with respect to the bearing surface. If the original surface is
obliterated entirely by overlapped dimple texturing, then it can be said
that 100% coverage area has been achieved. Additionally, the finished
surface texture should lack any sharp edges. It will be recognized,
however, that benefits accrue from finished coverage areas on the surface
of interest in excess of 60%, or more preferably between 70-90%, and on
up approaching 100%. The same percentage coverage can be used on both the
bearing surface and the sealing surface. Alternatively, a lighter
texturing can be used on the sealing surface as compared with the bearing
surface. As an example, a lighter texturing of 5-60% coverage can be used
on the sealing surface, while a heavier texturing of 60-100% coverage can
be used on the bearing surface.

[0055]In a preferred implementation, shot peening is chosen to form the
dimpled surface texture through topology modification. Shot peening
advantageously has characteristics of randomicity but with uniformity of
coverage. Shot peening is also a controllable process so that only those
desired surfaces will have a modified surface topography (this allows for
a machined surface to exist adjacent to a peened surface). As known to
those skilled in the art, shot peening is a cold working process in which
the surface of a part is bombarded by small spherical media called shot.
Each shot leaves a tiny dimple on the surface caused by impact. Shot
peening is more widely used to create compressive stress so as to reduce
fatigue crack. Inspired by the view that tiny dimples are generated on
the surface, shot peening is employed as described herein for a different
purpose in creating a dimpled surface texture which can constrain more
lubricant in the sealing/bearing surface contact zone(s) and generate
increased hydrodynamic pressure which better separates the
sealing/bearing surfaces.

[0056]Any one or more of the surfaces 20, 22, 30, 32, 40, 42, 50, 52 and
62 (or other gland conical, radial or cylindrical sliding surfaces)
described above can be subjected to the shot peening treatment. In a
preferred implementation, a two-step (dual) shot peening process is
utilized on the surface(s) of interest.

[0057]In a first step, shot peening is performed on the surface using a
first shot media at a first shot intensity. The shot peening action of
the first step is performed for a first period of time in order to obtain
a desired coverage area. The shot media is preferably cast steel which is
an exemplary implementation has a first average size of 0.011 inches, and
the intensity of the first step is 0.007˜0.010 C (measured by Almen
strip). Alternatively, the shot media is glass bead for softer surfaces
such as the inner cylindrical surface 22 of the first cylindrical
friction bearing 10 on the bushing 24 (with an intensity of the first
step being 0.008˜0.012N). In an exemplary implementation, the glass
media has an average size of 0.006 inches. The distinction between a hard
surface and a soft surface may be made based, for example, on whether the
hardness of the material exceeds a certain threshold (such as, for
example, a hardness of HRC 45). In the implementation described above for
the main journal bearing, the journal of hardened steel material has a
hardness of HRC 58-62, while the bushing of beryllium copper has a
hardness of about HRC 38.

[0058]Preferably for at least the bearing surface and possibly as well the
sealing surface, the peened coverage area resulting from completion of
this first shot peening step after the first period of time is between
60% and 100%. Coverage in excess of 100% may also be provided. Coverage
is defined as the extent (in percent) of complete texturing (for example,
dimpling) of the surface by the process step. Thus, with 100% coverage
the original surface texture of the surface which has been peened has
been obliterated entirely by the first shot peening step. Coverage in
excess of 100% is obtained by extending the exposure time to peening
beyond that time which is required to achieve 100% coverage. For example:
a 200% coverage would be achieved by shot peening the surface for twice
the amount of time necessary to obtain a 100% coverage. A lighter shot
peen resulting in coverage of 5-60% may alternatively be used for the
sealing surface.

[0059]FIG. 3 illustrates an exemplary impact pattern with respect to
execution of the first shot peening step with at or about 100% coverage
area.

[0060]FIG. 4 illustrates, in cross-section, a surface of interest which
has been treated by the first shot peening step.

[0061]Conversely, FIG. 5 illustrates, in cross-section, a surface of
interest which has been treated by the first shot peening step with a
coverage area of less than 100% (i.e., for a shorter period of time).
Such a peen may, for example, be used to effectuate the light texturing
used in one implementation for the sealing surface.

[0062]In a second step, additional shot peening is performed on the
surface (FIGS. 4 or 5) resulting from completion of the first step using
a second shot media at a second intensity. The shot peening action of the
second step is performed for a second period of time in order to obtain a
desired coverage area. The shot media is preferably cast steel having a
second average size of 0.011 inches (which is smaller than the first
average size), and the intensity of the second step is 0.007˜0.010
A (measured by Almen strip). Alternatively, the shot media is smaller
glass bead for softer surfaces such as the inner cylindrical surface 22
of the first cylindrical friction bearing 10 on the bushing 24.
Preferably, the peened coverage area resulting from completion of this
second shot peening step is between 60% and 100%.

[0063]FIG. 6 illustrates an exemplary impact pattern with respect to
execution of the second shot peening step with less than 100% coverage
area.

[0064]FIG. 7 illustrates, in cross-section, a surface of interest which
has been treated by the second shot peening step (when starting from the
first step result shown in FIG. 4) for a second period of time necessary
to obtain substantially 100% coverage area.

[0065]FIG. 8 illustrates, in cross-section, a surface of interest which
has been treated by the second shot peening step (when starting from the
first step result shown in FIG. 5). In this case, the second step has
less than 100% coverage (due to exposure for a shorted second period of
time). Again, such a lighter two step peen can be effectively used in one
implementation for the sealing surface as well as the bearing surface.

[0066]FIG. 9 illustrates, in cross-section, a surface of interest which
has been treated by the second shot peening step (when starting from the
first step result shown in FIG. 5). In this case, the second step has
100% coverage through selection of the requisite second period of time.

[0067]No matter what coverage area percentage is accomplished with the
second shot peening, it is preferred that the second shot peening step at
a minimum compact, as shown in FIG. 7, any sharp edges present in the
surface texture resulting from completion of the first shot peening step
(see FIGS. 4 and 5). This will result in an improved surface texturing
finish (beneficially reducing the possibility of metal-to-metal contact
in the bearing system, for example).

[0068]It is preferred with respect to at least the bearing surface that
following completion of the shot peening treatment (both or more steps)
of the surface of interest, that substantially 100% coverage area for
combined first and second step surface treatment with dimpling be
achieved. However, there are advantages to coverage areas of greater that
60%, 70-90%, and greater than 90%.

[0069]Although a two-step process is described, it will be understood that
the shot peening process could utilize more than two separate peening
actions. For example, a three-step, four-step, or more-step process could
be used. Each step would preferably utilize different average sized media
and different intensity levels.

[0070]For softer materials, for example at or below a hardness HRC45, only
one shot peening step action may be necessary. Harder materials, however,
benefit from the performance of two or more shot peening actions as
described above.

[0071]It will be understood that the cross-sectional surface texture
illustrations shown herein are schematic and exemplary in nature. The
illustrated regularity and periodicity of the dimple shape and location
shown in the FIGURES is not necessarily an accurate illustration of what
an actual shot peened surface would look like in cross-section but rather
is representative of the results achieved with the two step process. One
skilled in the art will understand the topologies which result from each
of the first and second steps given different respective first and second
periods of time for the peening action.

[0072]It is known in the prior art to provide radial and cylindrical
sealing/bearing system surfaces having a roughness of 8 to 16 microinches
Ra. This would comprise a typical polished surface of standard use (see,
also, FIG. 13). As a result of the completion of the surface treatment
process described herein, however, the shot-peened bearing surface of
interest will have a surface finish roughness greater than 20 microinches
Ra (see, also, FIG. 14).

[0073]Reference is now made to FIG. 10 which is an image illustrating
topography comparisons between surfaces that have been processed in
accordance with the two-step shot peening process described above to have
a surface roughness of greater than 20 microinches Ra (see, the bearing
shaft on the left) and surfaces with conventional surface roughness of 8
to 16 microinches Ra (see, bearing shaft on the right and bushing inner
surface at center). FIG. 11 is an image illustrating topography
comparisons between surfaces that have been processed in accordance with
the two-step shot peening process described above to have a surface
roughness of greater than 20 microinches Ra (see, the bearing shaft on
the left and bushing inner surface at center) and surfaces with
conventional surface roughness of 8 to 16 microinches Ra (see, bearing
shaft on the right). FIG. 12 is an image illustrating topography
comparisons between surfaces that have been processed in accordance with
the two-step shot peening process described above to have a surface
roughness of greater than 20 microinches Ra (see, the bearing shaft on
the right) and surfaces with conventional surface roughness of 8 to 16
microinches Ra (see, bearing shaft on the left).

[0074]FIG. 13 is an image illustrating the topography of a regular
machined surface such as would be used in the prior art for a bearing
system surface.

[0075]FIG. 14 is an image illustrating the topography of a two-step shot
peened surface as produced in accordance with the description above.

[0076]A conventional machined shaft with a conventional machined seal boss
(see surface of FIG. 13) and a shaft with a shot-peened seal boss (see
surface of FIG. 14) were tested on a test rig under the same operating
conditions. FIGS. 15 and 16 illustrate results of that testing and show
the beneficial effect surface texting (in general) and two-step shot
peening (in particular) in accordance with the process described above
has on the friction coefficient in these sealing system configurations.
The existence of the small dimples of the produced surface texture
generates hydrodynamic pressure, stabilizes or reduces friction. A shaft
and seal boss system with a regular machined finish (as known in the art)
shows a large variation in friction. For a shot-peened seal boss shaft,
however, the micro-dimples provide more reservoirs for grease to
lubricate the sliding surface with the seal. Meanwhile, the grease
contained in the dimples will generate hydrodynamic pressure to separate
the friction couple better. Therefore, the friction tends to be more
stable or reduced.

[0077]FIGS. 15 and 16 differ only in the manner with which the illustrated
data is identified and presented.

[0078]A conventional machined shaft with a conventional machined bushing
(see surface of FIG. 13), a shot-peened shaft (see surface of FIG. 14)
with a conventional machined bushing, and a conventional machined shaft
with a shot-peened bushing, were tested on a bearing test rig under the
same operating conditions. FIGS. 17 and 18 illustrate results of that
testing and show the beneficial effect surface texting (in general) and
two-step shot peening (in particular) in accordance with the process
described above has on the friction coefficient in these three bearing
system configurations. The existence of the small dimples of the produced
surface texture generates hydrodynamic pressure, stabilizes or reduces
friction. A shaft and bushing system with a regular machined finish (as
known in the art) is exposed to more asperity-to-asperity contact so that
the friction in this bearing system configuration shows a large
variation. For a shot-peened shaft and/or bushing system (two
implementations illustrated), however, the micro-dimples provide more
reservoirs for grease to lubricate the rubbing surfaces. Meanwhile, the
grease contained in the dimples will generate hydrodynamic pressure to
separate the friction couple better. Therefore, the friction tends to be
more stable or reduced.

[0079]FIGS. 17 and 18 differ only in the manner with which the illustrated
data is identified and presented.

[0080]Reference is now made to FIG. 19 which also illustrates the effect
shot peening in accordance with the two-step process described above has
on the friction coefficient in these three bearing configurations. FIG.
19 illustrates the same information as presented in FIGS. 17 and 18, but
the presentation is made in a different way. The FIG. 19 illustration
loses some information shown in FIGS. 17-18 concerning friction variation
in the regular head and bushing system and its deduction in a shot-peened
head and bushing system. Nonetheless, the frictional benefits of the
surface textured finishes for the bearing system as described herein are
evident.

[0081]In summary, a surface textured sealing system is presented for use
in a rock bit. Tiny dimples are created by a two-step shot peening
process on one or more surfaces of interest in connection with the
sealing system (for example, the seal boss or other sliding surface with
respect to the seal). The dimples of random distribution and non-uniform
size are formed over the surface of interest (for example with at least
60% coverage area for the bearing surface) and work as reservoirs to
constrain more lubricant in the surface contact zone. Hydrodynamic
pressure is generated in the dimple area and the seal friction is
reduced. Correspondingly, the sealing working condition is improved.

[0082]In summary, a surface textured head bearing is presented for use in
a rock bit. Tiny dimples are created by a two-step shot peening process
on one or more surfaces of interest in connection with the bearing system
(for example, in the main journal bearing). The dimples of random
distribution and non-uniform size are formed over the surface of interest
(at least 60% coverage area) and work as reservoirs to constrain more
lubricant in the surface contact zone. Hydrodynamic pressure is generated
in the dimple area and the bearing friction is reduced. Correspondingly,
the bearing working condition is improved.

[0083]Embodiments of the invention have been described and illustrated
above. The invention is not limited to the disclosed embodiments.